As a magnet type fan clutch which rotationally controls a cooling fan used for an internal combustion engine for a vehicle, a viscous fluid coupling which receives driving torque from a vehicle engine and transmits output driving torque to a radiator cooling fan and a method for controlling the viscous fluid coupling have been disclosed (see JP-A-9-119455). According to a mechanism of the viscous fluid coupling, a valve is opened and closed by deformation of bimetal which reacts to temperatures of air flowing through a radiator, thereby varying oil supply amounts and thus altering fan rotations. More specifically, this mechanism aims at prevention of fan noise and slip heat which are produced in a low idling condition at the time of vehicle stop, and comprises steps of: detecting a rotational speed of a vehicle engine; comparing the measured value with a limit value established in advance; detecting a rotational speed of a radiator cooling fan when the rotational speed of the engine is smaller than the limit value; comparing the fan speed with a predetermined fan speed limit value; and correcting an input signal to shift a valve member to a closing position when the fan speed is larger than the limit value.